1. Field of the Invention
This invention relates to a spark plug for use in an internal combustion engine in which a spark portion is discretely provided at a front end surface of a center electrode in registration with an outer electrode.
2. Description of Prior Art
With high output performance of an internal combustion engine, it has been required to ensure a positive ignition of a fuel, and at the same time, ensuring a spark-erosion resistant property for a spark plug. In order to comply with this requirement, an iridium-alloy based or platinum-alloy based tip is secured to a front end surface of a nickel-based center electrode by means of welding.
The tip enables positive ignition of the fuel and has good spark-erosion resistant properties. However, a thermal expansional difference between the tip and the center electrode causes results in a thermal stress therebetween, causing the tip to fall off the center electrode damaging a cylinder of the engine as the tip is alternately exposed to a heated and cooled environment while the engine is running.
Therefore, it is an object of the invention to provide a spark plug for use in an internal combustion engine which is capable of ensuring a positive ignition of a fuel and a spark-erosion resistant property at a high revolution range, and at the same time, securely preventing the spark portion from falling off the center electrode so as to contribute to an extended period of service life.